Add a pagination to a custom loop?

Hi, I would like to add a pagination to this loop that I got from a tutorial on the web, it works great for showing posts in the specified category but I’d also like for my visitors to be able to view older posts from the category. How do I do it with this loop?

<h1>A Piece of My Mind Opinion Posts</h1>
<?php
query_posts('posts_per_page=3&cat=121');
while(have_posts()) : the_post();
?>
<div class="grid_6 alpha omega opinion_posts">
<div class="grid_2 alpha">
<div class="thumbnail"><img style="width:140px; height:120px;" src="<?php echo get_post_meta($post->ID, "Thumbnail", true);?>" /> </div>
</div>
<div class="grid_4 omega">
<h1><a href="<?php the_permalink() ?>" rel="bookmark"><?php the_title(); ?></a></h1>
<p><?php the_excerpt(); ?><b><a href="<?php the_permalink() ?>">Read More</a></b></p>
</div>
<p class="clear"></p>
</div>
<?php endwhile; ?>

Thanks!